Recent advances in the development of semiconductor and organic light-emitting diodes (LEDs and OLEDs) have made these solid-state devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
An advantage of LEDs is that their turn-on and turn-off times are typically less than 100 nanoseconds. The average luminous intensity of an LED can therefore be controlled using a fixed constant-current power supply together with pulse width modulation (PWM), for example, of the LED drive current, wherein the time-averaged luminous intensity is typically linearly proportional to the PWM duty cycle. This technique of using PWM signals is disclosed in U.S. Pat. No. 4,090,189. Today, PWM is typically the preferred method for LED luminous intensity control in that it offers linear control over a range of three decades (1000:1) or more without suffering power losses through current-limiting resistors, uneven luminous intensities in LED arrays, and noticeable colour shifts as identified by A. Zukauskas, M. S. Schur, and R. Caska, 2002, Introduction to Solid-State Lighting. New York, N.Y., Wiley-Interscience, p. 136. The PWM signals used to control the LEDs are preferably generated by microcontrollers and associated peripheral hardware.
An application of LEDs is in theatrical lighting fixtures. These fixtures are commonly controlled using an industry-standard asynchronous serial communications network protocol referred to as “DMX512.” This protocol was introduced in 1990 by the United States Institute for Theatre Technology (USITT) and is presented in their publication, “USITT DMX512/1990 Digital Data Transmission Standard for Dimmers and Controllers”. Most theatrical lighting manufacturers quickly adopted the DMX512 protocol as an industry standard.
As originally designed, DMX512 was intended primarily to control up to 512 lighting fixture dimmer controls. To this end, the protocol comprises a serial stream of data transmitted over an RS-485 multidrop serial communication link, wherein each data “packet” comprises a packet header, an 8-bit START code, and 1 to 512 8-bit data bytes called “frames.” FIG. 1 illustrates a timing diagram for control of a lighting system using the DMX512 protocol. When the start code is NULL, the data bytes are interpreted as dimmer control settings, thereby allowing up to 256 discrete intensity levels for each lighting fixture. The position of each frame within the packet defines the DMX512 “address” of the lighting fixture. In operation, the lighting fixture receives each packet and extracts the data frame corresponding to its fixed DMX512 address, as illustrated in the configuration of FIG. 2.
Where a theatrical lighting fixture has additional components such as color filter wheels, moveable lenses and irises, or motorized mounts, the lighting fixture may have multiple DMX addresses (referred to as DMX “channels”) to independently control these components.
The DMX512 protocol also makes provision for 255 optional START codes wherein the START code value is between 1 and 255, which the USITT publication cited above states are “for future expansion and flexibility.”
The Entertainment Service and Technology Association released a document describing a proposed successor of the DMX512/1990 protocol, “Draft BSR E1.11, Entertainment Technology—USITT DMX512-A Asynchronous Serial Digital Data Transmission Standard for controlling Lighting Equipment and Accessories, Revision 3” in 2000. The purpose of this document is primarily to more precisely define the scope of the USITT DMX512 serial network protocol and related RS-485 network physical layer. It also however formally defines Alternate START Codes wherein the START code is an 8-bit value other than NULL. Annex E of this document defines reserved Alternate START Codes for special purposes and future development of the draft standard. These include a Manufacturer ID code intended to identify proprietary data packets and a System Information Packet intended to identify manufacturers' specific products.
Both DMX512/1990 and its proposed successor DMX512-A are real-time lighting fixture control protocols in that each data frame represents the current intensity for the lighting fixture dimmer, wherein each data packet is transmitted at least 44 times a second in accordance with the DMX512 protocol timing requirements.
With recent advances in LED lighting technology, it has become desirable to execute complex lighting control sequences using the DMX512 protocol and RS-485 asynchronous serial communication. Some of the problems, however, with using the existing DMX512 protocols and RS-485 asynchronous serial communication in a manner for both communication and synchronization of a networked ensemble of lighting fixtures whose operation comprises complex and synchronized lighting control sequences, are that the DMX512 protocol does not support phase shifting of the data stream, nor does the protocol support multiple interpretations of the data stream. Furthermore, the DMX512 protocol does not support the transmission of hierarchical data, remote querying of a dimmer control address, nor autonomous synchronized color fading. In addition, a problem with synchronized strobing of light sources is that timing inaccuracies can cause perceptible differences in light output.
It can be advantageous to address logical groups of lighting fixtures and using the current state-of-the-art in DMX512 technology, this requires that the DMX512 master controller maintain a list of DMX512 addresses assigned to each group, and to sequentially address each of the lighting fixtures in a DMX512 color packet. The disadvantage of this approach is that the DMX512 packet can take up to 44 milliseconds to transmit in its entirety. In the worst case scenario therefore, there is a corresponding delay of 44 milliseconds between lighting fixtures in the same logical group responding to a common command. This delay was not of particular importance when the DMX512 protocol was introduced in 1990, as almost all theatrical light sources consisted of incandescent lamps with response times measured in tens of milliseconds. Delays in responding to a common command were therefore mostly imperceptible. However, the introduction of high flux light-emitting diodes suitable for entertainment and architectural applications has reduced the response time from tens of milliseconds to tens of nanoseconds. Consequently, delays of even a few milliseconds between LED-based lighting fixtures responding to a common command can be noticeable and objectionable.
Furthermore, it is common practice with theatrical lighting systems to assign DMX512 addresses to lighting fixtures after they are installed, often in relatively inaccessible locations. If an error is made in assigning a DMX512 address, it becomes a trial-and-error process to determine why the lighting fixture is not responding to the DMX512 master controller.
Furthermore, it is known that perceived brightness of LEDs has a non-linear relationship to the radiometric intensities of the LEDs, including for example the Helmholtz-Kohlrausch effect and Bezold-Brücke phenomenon. This relationship between perceived brightness and radiometric intensities is described by, for example, Wyszecki, G., and W. S. Stiles in “Color Science: Concepts and Methods, Quantitative Data and Formulae,” New York, N.Y.: Wiley-Interscience, 2000. This relationship results in a perceived non-linear brightness when using linear control parameters. The relationship between perceived lightness and measured illuminance of an object can be approximately represented by Steven's Law defined as follows:B=αL0.5  (1)
where B is the perceived lightness, α is a scaling constant, and L is the luminance (measured in candela per square meter per steradian) of the illuminated object at a given point on its surface.
Stevens' Law (Equation 1) has been used in theatrical lighting fixture dimmers to linearize the relationship between the raw color represented by, for example, a dimmer control panel slide resistor or DMX512 color packet frame values and the perceived lightness of illuminated surfaces as described in IESNA, 2000, IESNA Lighting Handbook, Ninth Edition. New York, N.Y., Illuminating Engineering Society of North America, where it is commonly referred to as “square law dimming.” However, this approach assumes a single light source for which only the intensity can be varied as described in U.S. Pat. No. 5,309,084, for example. Therefore, problems arise when attempting to apply this technique to multicolor light sources, such as for example a light fixture with red, green, and blue LEDs, whose intensities can be independently or interdependently varied.
Thus, there is a need for a solution that allows the DMX protocol to be used for control of lighting fixtures which can overcome the problems identified in the prior art.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.